Method and device for weed control

ABSTRACT

Weed seeds are controlled by high voltage pulses with short duration which electropermeabilize the cell membranes of weed seeds in the ground. The device is selective and damages only germinating weed seeds and plants early in their life cycle. The required amount of energy is small; with rectangular pulses the optimal field strength is between 100-300 kV/m with a duration of (10-100 microseconds). A transformer placed on a sowing machine transforms electrical energy to high voltage pulses. The energy may be taken from the pulling tractor via a transmission or from an integrated power source. The high voltage pulses are applied to electrically conducting via applicators to two or more fixedly spaced plates to the soil around newly sown seeds.

BACKGROUND OF THE INVENTION

This invention concerns a method and a device for weed control by meansof electricity.

For many years the use of weed poisons, in addition to purely mechanicalmethods, has been the dominating method for weed control. There isincreasing concern that an extensive use of weed poisons may harm theenvironment as well as humans and animals. This concern has led to anincreasing interest for alternative non-chemical methods as a complementto the traditional mechanical methods.

The concept of killing weed by means of electric energy has been usedsince the 1890's when “Vegetation exterminators” was patented (Sharp, A.A. 1893, Scheible, A. 1895) The technique was also developed also forweed control in the agriculture (Buret, W. E. 1928, Opp, F. W. ;Collins, W. C. 1935, Opp, F. W. 1952, Poynor, R. R. 1954). It has beenfurther suggested for use in farming, in particular non-specificsuperficial treatment of the soil by moving electrodes along the groundsurface and applying an alternating voltage (Laronze 1982) or constanthigh voltage (Krause 1975).

Equipment using electric current for pruning of blueberry bushes anddehydration of the foliage of root crops before harvest has beensuggested by (Plueneke, R. H. ;Dykes, W. G. 1975, Plueneke, R. H.;Dykes, W. G. 1977).

Thinning out of drilled plants was suggested in the 1950's by (Rainey,E. C. 1952, McCreight, N. L. 1953). Use in soil sterilizing andfertilizing has also been suggested (Keller, C. R. 1947, Opp, F. W.;Collins, W. C. 1935). Wayland et al (1975) showed that technically onecan use micro waves for sterilizing soil before sowing. Weed and weedseed a few cm into the ground can be killed with microwaves having afrequency of 2450 MHz and the power 60 kW. The machine used was a 150 kWdiesel driven generator that fed a microwave generator and a hydraulicsystem that slowly moved the equipment forward. Dipose et al. (1984)estimated that it would take between 100-1000 hours per feature to treatsome types of weeds. Such long times are unavoidable. The machineradiates microwaves down into the ground to heat everything and mustliterally boil the weed which requires a great deal of energy. A furtherdrawback is that also the micro flora in the soil is knocked out.

As is apparent from the above it has not yet turned out to be practicalto use electricity for weed control. It is quite simply too expensiveand time consuming in relation to chemical and mechanical controlmethods.

THE SUMMARY OF THE INVENTION

Despite the above problems the object of the invention is to provide amethod for weed control based on the use of electricity. Surprisinglyenough, in view of the known technique, weed control with electricitycan in accordance with the invention be carried out with a veryreasonable energy consumption by using short high voltage pulses withlow energy content but sufficiently high for electropermeabilitation ofcell membranes in growing weeds. The invention can be used for weedcontrol in farming, horticulture gardening, forestry, parks or othersuitable areas and particularly connection with sowing. The inventiononly damages growing seeds and plants early in their life cycle andmakes it impossible for them to germinate or survive on. Dry seeds forsowing are not affected which is why the invention can be used withadvantage in connection with sowing without influencing seedgermination. Neither is the micro flora in the soil affected within thespecified voltage span. When seeds start to grow they are very sensitiveto electricity. However, when the seed has grown into a plant aconsiderable amount of electrical energy is necessary to harm the plantto the extent that it does not continue to grow on. In order harmungrown seeds with electric voltage pulses or electric current,considerable amounts of electric energy is required, one mustpractically boil the seed.

The problem is that when the intended crop is sown, the seeds of theweeds are already in the ground and have already started to germinate.According to known techniques, one is therefore obligated to fight theweeds in order to let the intended crop come into its own. By treatingthe ground that is to be sown in accordance with the invention withelectrical pulses with low energy, weeds that have just started togerminate can be stopped in their development. By undertaking thiscontrol simultaneously with sowing, the intended crop is given animproved chance of having time to grow before the weeds grow. Thus, byemploying the use of electricity in accordance with the presentinvention in connection with planting and preferably simultaneously withsowing, it is possible to eliminate germinating weed seeds. Weed seedsgrowing later have less chance of asserting themselves since they havecompetition from the growing crop. Furthermore, since sowing typicallyhas been preceded by mechanical working of the ground, pre-existingweeds already would have been eliminated. By feeding short high voltagepulses to the ground the sprouting weed can be controlled. The highvoltage pulses of short duration will not cause any great damages to theweed sprouts as such. However, the amount of energy is sufficient toachieve an electropermeabilizing of the cell membranes of the sproutingweed seeds. This in turn leads to stopping growth for the damagedsprouts. Since the seeds of the intended ground crop has yet started togerminate at sowing they are not affected and sowing and weed controlcan be executed with the same machine without any inconvenience, on thecontrary it is particularly practical to execute weed control and sowingsimultaneously so that the intended crop gets the competing weed sproutseliminated at the optimally correct time. The amount of energy that isrequired for the control described here is insignificant and is apparentfrom the following formula 1.

Formula 1

Mean power=(conductivity in the ground)×(the field strength)²×(degree ofmodulation)/(Density) (W/kg)

Conductivity=0.2 siemens

Field strength=300 000 volt/m

degree of modulation=1/10 000

density 1500 kg/m³

The momentary power in the pulse is very high, for instance 12 MW/kg butsince the pulse is of very short duration (typically 100 microsecondslong) and is applied once a second the mean power is considerably lower,that is 12 kW/kg. In practice a capacitor battery charged that isdischarged very quickly and deliver the desired high voltage pulse.Alternatively, alternating voltage is used that can be transformed tohigh voltage.

The treatment of plant cells with these high voltage pulses primarilyinfluences protoplasm dependent on the barrier that is constituted bythe cell wall. With rectangular pulses the optimal amplitude of thefield strength is between 100-300 kV/m with a duration of (10-100 microseconds). With exponentially declining pulses the field strength isweaker (25-75 kV/m) and the duration of the pulse longer (1000-20 000microseconds).

There are examples of experiments with intact plant cells of forinstance rice that shows that the cell wall is not an absolute obstacleto electroporation (Morikawa et al 1986; Dekeyser et al.,1990). Theinvention is based on one using an electric field strength high enoughso that the cell membranes are destroyed or punctured and the cell diesas a result of this. The earth bed is treated with pulses of shortduration (0,1-1 millisseconds) with the field strength 150-300 kV/m(1500 3000 V/cm). These high field pulses have has a selectivedestructive effect on germinating weed seeds while dry seeds for sowingare not influenced. In order to effect the bacterial flora in theground, higher field strengths and longer pulses are necessary. However,larger animals as maggots or noxious; insects may be affected.

This new technique has been evaluated with white mustard (Sinapsis alba,var. Emergo) simulating weed with high germination (87 %). The whitemustard was cultivated in fertilized peat earth (70% earth, 30% Perlite)in plastic containers (5×11×16 cm) that were placed on a watering carpetin order to ensure an even earth humidity. The plants were cultivated ina climate chamber with a mean climate corresponding to 15th May inSk{dot over (a)}ne. The evaluation was executed in three main groups, I.dry seeds, II. germinated seeds and III seeds moistened in nutritionsolution. In all tests pulses with a pulse length of 80 μS have beenapplied. At all tests check groups have been used where the treatmenthas been as like that of the treated groups as possible.

The dry seeds (group I) were placed in trays with nutrient solution, andpulses with a field strength of 500-7500 V/cm was applied. Groups of ca5 seeds were treated and were then placed on moist growing substrate inpetri bowls. The seeds germinated normally, that is the treatment had noeffect.

The germinating seed (group II) were tested in cultivating trays, 15×25cm, with ca three cm thick layer of soil. Groups of 6 seeds were sownand treated with 1000, 1500, 2000, 2500 and 3000 V/cm. One seed in everytray constituted control. The seeds were treated 2, 4, 6 and 8 daysrespectively after sowing, in each group totally 8 cultivating trayswere included, where one tray was control. The results were evaluated bydetermining of the number of seeds not germinating by visual examinationof and by weighing of the plants. Our evaluation showed that a treatmentwith field strengths above 2000 V/cm had a strong effect on seeds thatwere treated 2 and 4 days after sowing. If the plant was left 6 or 8days a clear retardation could be observed but the plant lived on.

In order to establish if the moisture content of the seed was ofdecisive importance to the result, seeds were placed in nutritionsolution for different times (group III). All were treated with 3000V/cm. The tests showed that all the seeds that had been in a nutritionsolution for at least 35 minutes lost there fertility entirely.

The method has no effect on dry seeds. Also plants that have reached acertain size can take the treatment without serious damages. The methodmay be very useful particularly in connection with sowing where theuseful crop is not damaged but weeds that are in an early stage aretotally knocked out. Already established weeds can be handled withmechanical methods.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of a device in accordance with the invention placed on asowing machine pulled by a tractor is shown in FIG. 1.

FIG. 2 shows the sowing machine with the invention in more detail.

FIG. 3 shows a sowing share with applicators that introduce voltagepulses into the soil.

FIG. 4 is an example of how the pulse generator may be constructed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In accordance with FIG. 1 a transformer (2) is placed on a sowingmachine, that is drawn by for instance a tractor. The transformerconverts electrical or mechanical energy into high voltage pulses. Theenergy may be taken from the pulling tractor via a transmission (1) orfrom an integrated power source. The high voltage pulses are applied viaapplicators (3) placed on the sowing share down to the soil around thejust sown seed.

In accordance with FIG. 2 the transformer (2) is coupled via a number,at least 2, high tension wires (5) to electrically conductingapplicators (3), that have a given relative spacing and are in contactwith the soil down to a depth of 4-15 centimeters, typically 5centimeters. A sensor system (7) placed on the applicators provides thetransformer with information of the conductivity of the soil and thespeed with which these move through the soil. The signals are fed backto a signal transformer (8) placed in the generator system and are usedto synchronize the pulse generator so that each part of the soil istreated with 5-25 pulses, typically 10. The pulses are to have a shortduration, typically 0,01-1 millisecond and a field strength of 150-300kV/m. At a distance between the applicators of 5 centimeters the voltageis typically to be 7500-15000V.

FIG. 3 shows the sowing share in more detail, the seed pass through achannel (9). The sown seed (10) gets into a groove (12) in the soil (11)between a pair of applicators (3). The applicators may be several innumber and may be shaped as plates, rods or discs that are rigid or canrotate but these always have a constant relative spacing. On theapplicators or in connection with these there are sensors (7) thatregister movement through the soil and the conductivity of the soil.

In FIG. 4 is shown a block diagram of the pulse generator. The energy orpower transformer (15) gets energy from the tractor or an integratedpower source. The energy is transformed for instance by means of atransformer and rectifier (16) to a suitable level for instance 24 V DC.This rectified voltage provides current to a high voltage generator (17)for instance built as an oscillator and transformer. The high voltagemay possibly be stored in a number of reservoir capacitors afterrectifying. A pulse shaper (18) generates short pulses by opening andclosing a valve element (tyristor, transistor or the like) with veryshort duration. The valve element can work directly with the highvoltage or with a lower voltage that is transformed up in a last pulsetransformer before it is applied in the soil with the applicators (19).The sensors (20) placed on the sowing share are coupled to signaltransformers (21) giving suitable signals to a control logic (22) thatcontrols the high voltage generator by controlling amplitude, frequencyand/or pulse length. The control logic is also connected to the pulseshaper and controls the duration of the pulses and frequency andpossibly pulse shape. Protections (23) may be used to protect usersagainst electric casualties. If the user comes within the operating areaof the device power trandnission, control logic and/or pulse shaper canbe disconnected to prevent delivery of pulses.

In a further embodiment of the invention the control of weeds may befurther effected by the adding of some suitable substance that effectsthe cells, for instance calcium ions, in connection with the electricalweed control. The added substance penetrates through the perforated cellmembranes and work inside the cell restraining its growth or killing thecell. The amount of substance that has to be added is comparativelysmall and for instance, in the case of calcium ions no residualpoisonous effect whatsoever, for the crop, rather the opposite. At thesame time the efficiency in the electrical control is increased andlesser amounts of electricity are needed. Tests indicate that therequired applied field strengths can be reduced to approximately ⅓. Ifthe calcium ions are already present in sufficient amounts in theground, no admixture has to be done. One can consider adding thesubstance in some form of solution, for instance CaNo₃, that is sprayedover the ground. Alternatively that the substance may be added togetherwith the crop that is to be sown. If the substance used is constitutedby a conventional pesticide the concentration of this can be reducedradically through elektroperforation since efficient influence may takeplace in the cells of the plant. The adding may take placesimultaneously with or just before the electroperforation so that thesubstance can pierce through the cell membranes of the weed germs andwork inside the cell. The substance can be a mineral salt or an organiccombination, as a pesticide in low concentration. A concentration ofcalcium ions in the area 10-100 millimoles may be suitable. In order toachieve an exchange that is as good as possible from the used electricenergy the electric pulses should have an exponentially rising frontflank and an exponentially falling rear flank. The duration may bebetween 10 and 10 000 microseconds.

While the foregoing invention has been described as being usedcontemporaneously with sowing, it also may be used before sowing or evenwith connection with an earlier mechanical preparation of the ground.

The invention also may be used after sowing provided that it is adaptedso that the intended crop is not damaged. For instance one can waituntil this has grown sufficiently not to be harmed, alternatively onecan at spaciously planted crops arrange applicators in pairs betweenrows of crop and feed these electrically so that the crop is notsubjected to any pulses (the crop is surrounded by applicators with thesame voltage).

What is claimed is:
 1. Method for electrical weed control, whichcomprises feeding electrical high voltage pulses to the ground of anelectric field sufficient to electropermeabilize growing cell membranesof weed seeds, wherein the electrical field strength is 100-300 kV/ mand every pulse has a duration between 10-10,000 microseconds (10⁻⁶sec).
 2. Method for electrical weed control, which comprises feedingelectrical high voltage pulses to the ground of an electric fieldsufficient to electropermeabilize growing cell membranes of weed seedswherein each section of ground being treated is subjected to 5 to 30pulses.
 3. Method for electrical weed control, which comprises feedingelectrical high voltage pulses to the ground of an electric fieldsufficient to electropermeabilize growing cell membranes of weed seeds,wherein the pulses have an exponentially rising front pulse flank and anexponentially falling rear pulse flank.
 4. Method for electrical weedcontrol, which comprises feeding electrical high voltage pulses to theground of an electric field sufficient to electropermeabilize growingcell membranes of weed seeds, and further including the step of feedinga cell poisonous substance to the ground whereby said electrical highvoltage pulses drive said cell poisonous substance through said cellmembranes of said weed seeds whereby to kill the cell.
 5. A methodaccording to claim 4, wherein said cell poisonous substance comprisescalcium ions.
 6. Method for electrical weed control, which comprisesfeeding electrical high voltage pulses to the ground of an electricfield sufficient to electropermeabilize growing cell membranes of weedseeds, wherein the pulses are fed to the ground simultaneously withsowing of a crop.
 7. Device for selective weed control comprising, incombination, applicators that are connected to a pulse generator andthat receive electrical energy from an energy transformer, saidapplicators being arranged in pairs and being of an electricallyconductive material, said applicators being arranged to contact the soilthat is to be treated against germinating weeds, a device that registers(1) movement of the applicators in relation to the soil, and (2)conductivity of the soil, said applicators being spaced apart from oneanother, said pulse generator being for generating pulses with a certainlength and shape, said pulse generator being provided with a monitorthat monitors energy transfer in amplitude, pulse length and frequency,and a sensor for protection of personnel.
 8. Device according to claim7, wherein the applicators are shaped as fixed plates.
 9. Deviceaccording to claim 7, wherein applicators being shaped as rotatingwheels.
 10. Device according to claim 7, wherein applicators are adaptedto penetrate into the soil to a depth corresponding to sowing depth. 11.Device according to claim 7, wherein the applicators are adapted totransfer electric energy through direct contact with the soil to thedepth of the weed seeds.
 12. Device according to claim 7, wherein theapplicators are adapted to transfer electrical energy to the soil viacapacitive or inductive coupling.
 13. Device according to claim 7,wherein the pulse generator is adapted to deliver pulses with a durationof 0,01-10 mS.
 14. Device according to claim 7, wherein the pulsegenerator is adapted to deliver pulses in the shape of pulsatingrectified voltage.
 15. Device according to claim 7, wherein the pulsegenerator is adapted to deliver pulses in the form of pulsatingalternating voltage.
 16. Device according to claim 7, wherein the pulsegenerator is adapted to deliver pulses with sufficiently high amplitudeto induce an electric field strength in the soil sufficient enough todestroy the germination of weed seeds in the range of 75-500 kV/m. 17.Device according to claim 7, and further comprising a device forsynchronizing the pulses with movement of the applicator so that eachregion of soil is treated with 5-30 pulses.
 18. Device according toclaim 7, and further comprising a sensor for measuring conductivity ofthe treated soil.
 19. Device according to claim 7, and furthercomprising a sensor for measuring movement of the applicator in thetreated soil.
 20. Device according to claim 7, and further comprising acontroller for controlling voltage amplitude and current.
 21. Deviceaccording to claim 7, and further comprising a controller forcontrolling voltage amplitude, pulse duration and number of pulses basedon sensed conductivity and/or movement.
 22. Device according to claim 7,and further comprising cover sheets, for the applicators with built incontact breakers that deactivate supply of voltage to the applicators inthe event the cover sheets are touched.
 23. Device for selective weedcontrol comprising high voltage applicators that are connected to apulse generator that receives electrical energy from an energytransformer, wherein the applicators are formed of electricallyconductive material and are arranged in pairs spaced from one another,said applicators being arranged to make contact with the soil that is tobe treated against germinating weeds, and a device that registersmovement of the applicators relative to the soil, and conductivity ofthe soil, said pulse generator being arranged to generate pulses with aset length and shape, said pulse generator being provided with a monitorthat controls energy amplitude, pulse length and frequency, and a sensorfor protecting personnel.
 24. Device according to claim 23, wherein theapplicators are shaped as fixed plates.
 25. Device according to claim23, wherein applicators being shaped as rotating wheels.
 26. Deviceaccording to claim 23, wherein applicators are adapted to penetrate intothe soil to a depth corresponding to sowing depth.
 27. Device accordingto claim 23, wherein the applicators are adapted to transfer electricenergy through direct contact with the soil to the depth of the weedseeds.
 28. Device according to claim 23, wherein the applicators areadapted to transfer electrical energy to the soil via capacitive orinductive coupling.
 29. Device according to claim 23, wherein the pulsegenerator is adapted to deliver pulses with a duration of 0,01-10 mS.30. Device according to claim 23, wherein the pulse generator is adaptedto deliver pulses in the shape of pulsating rectified voltage. 31.Device according to claim 23, wherein the pulse generator is adapted todeliver pulses in the form of pulsating alternating voltage.
 32. Deviceaccording to claim 23, wherein the pulse generator is adapted to deliverpulses with sufficiently high amplitude to induce an electric fieldstrength in the soil sufficient enough to destroy the germination ofweed seeds in the range of 75-500 kV/m.
 33. Device according to claim23, and further comprising a device for synchronizing the pulses withmovement of the applicator so that each region of soil is treated with5-30 pulses.
 34. Device according to claim 23, and further comprising asensor for measuring conductivity of the treated soil.
 35. Deviceaccording to claim 23, and further comprising a sensor for measuringmovement of the applicator in the treated soil.
 36. Device according toclaim 23, and further comprising a controller for controlling voltageamplitude and current.
 37. Device according to claim 23, and furthercomprising a controller for controlling voltage amplitude, pulseduration and number of pulses based on sensed conductivity and/ormovement.
 38. Device according to claim 23, and further comprising coversheets for the applicators with built in contact breakers thatdeactivate supply of voltage to the applicators in the event the coversheets are touched.